Membrane switches have become popular for use in industrial controls, home appliances, office equipment and automotive applications. Membrane switches are touch activated and have some advantages over electrical switches. For instance, mechanical switches are prone to mechanical failure due to breakage of moving parts. This problem is not prevalent with membrane switches. In addition, membrane switches conserve considerable space over conventional mechanical switches due to their very thin profile.
A typical membrane switch includes a lower insulating layer, a dielectric spacer layer and an upper insulating layer. Typically, one of the insulating layers is fixed and the other insulating layer is movable, with the spacer layer separating the two insulating layers. One or more electrically conductive contact areas are provided on the upper face of the lower insulating layer and the lower face of the upper insulating layer in facing relation to define the contacts of the switch. Most often, a plurality of pairs of the facing contact areas are provided between the juxtaposed insulating layers, and the spacer layer has a plurality of openings through which the contact areas of each pair thereof are exposed to each other in facing relation.
To make connection with a device to be switched, the facing contact areas are extended to the periphery of their respective insulating layers and are connected to external leads which, in turn, are connected to the particular device to be switched. In use, movement of the insulating layers toward each other, such as moving the movable insulating layer toward the fixed insulating layer, causes a given pair of the facing contact areas to engage and close a circuit through the external leads connected to the device to be switched.
While membrane switches have been fairly easy and efficient to fabricate, incorporating the switches into various apparatus has caused problems, particularly when the fabrication involves the application of heat and/or pressure. The insulating layers which carry the facing contact areas of the membrane switch typically are fabricated of some type of plastic material, such as polyester. When these layers are subjected to heat and/or pressure, they can collapse, even when spacer layers are used in the switch. Collapsing of the insulating layers causes the distance between the facing contact areas to vary and can even cause the contact areas to touch and cause permanent electrical contact.
One example of the manifestation of the problem discussed above is switches mounted in a steering wheel for a vehicle horn in the automotive industry. To facilitate access to the actuator for the horn switch, the actuator is preferably disposed in the central surface of the steering wheel. The advent of the air-bag has made it common to locate an air-bag assembly in the center of the steering wheel thereby complicating the task of mounting switches in steering wheels. The air-bag assembly is relatively bulky, comprising a compacted air-bag and a gas generator. In order to locate the horn switch assembly with the actuator disposed in the central surface of the steering wheel, mechanical horn switches have been incorporated into the air bag assembly. Such mechanical horn switches are attended by complexities of increased weight and volume. As an alternative, horn switches are sometimes placed beside the air-bag assembly, with the actuator for the horn switch located outside the central surface of the steering wheel. Actuators for such switches are often difficult to access in sudden situations.
The use of a membrane switch in a steering wheel for actuating the vehicle horn would solve the problems posed by mechanical switches, described above. The membrane switch could provide a significantly larger area of actuation, and the membrane switch would present considerably fewer volume and mass problems than the mechanical switches. However, the use of a membrane switch poses its own problems because of the application of heat and pressure during manufacture of the steering wheel assembly, such as during overmolding processes.
The present invention is directed to solving the above myriad of problems by providing a method of fabricating a membrane switch, an article of manufacture and the switch, itself, wherein the components of the switch are protected from collapsing during manufacture, such as during processes involving the application of heat and/or pressure.